This is an interdisciplinary research project between the departments of Neuroradiology, Neurological Surgery and Head and Neck Surgery. During the previous grant period we have developed several extremely low viscosity silicone rubber formulations for the embolization of vascular tumors and malformations. Recent materials include 4 centistoke silicone rubber, one variety of which can be made to foam and expand to twice its volume upon polymerization. During this year we tested histological toxicity in the rat and studied the results with the scanning electron microscope. No adverse effect was found after a period of 6 weeks. The intimal cells kept their normal surface appearance. The technique of inclusion of cytotoxic materials in the silicone rubber for subsequent injection into tumors is being developed.